One type of image display apparatus which displays an image by diffracting coherent light using a diffraction grating pattern is an image display apparatus which uses a computer generated hologram (hereinafter called “CGH”).
In this image display apparatus, a diffraction grating pattern determined by a computer on the basis of an input image corresponding to a target display image that is to be displayed is displayed on a phase modulation type of liquid crystal panel, or the like, and by irradiating laser light onto the liquid crystal panel so as to cause diffraction, a wave front of display light from a virtual image position is reproduced and a virtual image is displayed to a user. As a result of this, a characteristic feature of the CGH method is that a three-dimensional stereoscopic image can be displayed at a position in front of or behind the liquid crystal panel.
For example, one image display apparatus which displays a three-dimensional stereoscopic image by a CGH method is a three-dimensional scene holographic reconstruction apparatus (see Patent Document 1). Furthermore, although it does not involve a CGH method, there is also an image display apparatus which displays a three-dimensional stereoscopic image to a user by a diffraction grating pattern (see Patent Document 2).
In the image display apparatus which uses the CGH method described above, although a large amount of research has been carried out into a three-dimensional image display method which produces a natural three-dimensional effect, such a method has not come into common use yet, for reasons such as the fact that it requires a laser light source and a spatial modulation element having a small pixel pitch and a large number of pixels. However, in recent years, liquid crystal panels for a projector have achieved higher definition, and furthermore higher output has been achieved in visible-range semiconductor lasers and an image display apparatus using a CGH method has come close to practical application.
As described above, a conventional image display apparatus using a CGH method irradiates laser light onto a spatial modulation element which is displayed in superimposed fashion with a diffraction grating pattern on respective pixels of an input image, and displays an image by laser light diffracted by the displayed diffraction grating pattern.
Here, when displaying an input image in which a large number of pixels of high brightness are present, a plurality of diffraction grating patterns are superimposed, and therefore the respective diffraction grating patterns are unclear. As a result of this, the diffracted light on the displayed pixels is reduced and pixels are displayed at a lower brightness than the brightness that should be displayed. Conversely, when displaying an input image in which there is only a small number of pixels of high brightness, the extent of superimposition of the diffraction grating patterns is small, and therefore the diffraction grating patterns of the respective pixels are displayed clearly. As a result of this, the diffracted light on the displayed pixels is increased and pixels are displayed at a higher brightness than the brightness that should be displayed.
From another perspective, the diffraction efficiency of laser light irradiated onto the spatial modulation element does not change greatly with the input image, and the sum total of the brightness of the whole virtual image does not change greatly. Therefore, if the input image contains a large number of pixels having a high brightness, the laser light is distributed to a large number of pixels and the pixels of the virtual image are displayed at low brightness. On the other hand, when the input image only contains a small number of pixels of high brightness, then the laser light is concentrated in a small number of pixels and the pixels of the virtual image are displayed at a higher brightness. In this way, when displaying an image by a computer hologram, since the diffraction efficiency is virtually uniform, then if there is a small number of bright pixels in the input image, the brightness becomes higher than the brightness that should be displayed, and conversely, if there is a large number of bright pixels, the brightness becomes lower than the brightness that should be displayed.
Furthermore, by using laser light sources of three types, a red, green and blue light source, and superimposing and outputting the images of the respective colors, it is possible to achieve a full color image display, but when a full color image display is performed, then color deviations occur in the virtual image if there is variation in the number of pixels having high brightness in the input images of the respective colors, as described above.
In this way, in a conventional image display apparatus which uses a CGH method, when performing a moving image display, the number of pixels having high brightness in the input image changes, and therefore the brightness and display color change with the scene and similarly, when displaying an information image such as text characters, the number of pixels of high brightness in the input image changes with the number of text characters, and hence there is a problem in that the brightness and display color change, and so on.
Patent Document 1: Japanese Translation of PCT Application No. 2008-541145
Patent Document 2: Japanese Patent Application Publication No. 6-202575